The HIV-encoded Vpu protein is an integral membrane phosphoprotein which is present in high concentrations in virus-producing cells but appears to be excluded from virus particles. Vpu forms homopolymeric complexes in vivo and in vitro; protease protection experiments indicate that Vpu possesses the topology of a Class I integral membrane protein but lacks the signal peptide processing signal. An analysis of potential interactions of Vpu with other viral or cellular proteins demonstrated that it affected the processing of the HIV envelope glycoprotein precursor gp160 and the intracellular stability of CD4. In the absence of Vpu, gp160 and CD4 form stable intracellular complexes which cannot be transported intracellularly. In the presence of Vpu, gp160/CD4 complexes are destabilized and normal synthesis of gp120/gp41 ensues. A series of pulse/chase experiments, designed to investigate the mechanism of the Vpu-dependent destabilization of gp160/CD4 complexes revealed that Vpu induces the rapid degradation of CD4 reducing its half-life from normally 6 hours to 12 minutes. Degradation was shown to depend on the retention of CD4 in the endoplasmic reticulum of the cells which is accomplished by the formation of gp160/CD4 complexes. However, gp160 is not directly involved in the degradation of CD4 and its function as a CD4 trap can be substituted by Brefeldin A, a drug that blocks membrane traffick in the endoplasmic reticulum.